The electrical characteristics of many semiconductor devices are significantly improved if the semiconductor material from which they are fabricated is of single crystal structure rather than of polycrystalline structure. This is especially true in the case of devices whose performance improves with increasing minority carrier lifetime.
It is well known that the boundaries between grains in polycrystalline semiconductor material are often populated by recombination centers for mobile charge carriers, and are thus effective in reducing minority carrier lifetime. Solar cells comprise one type of semiconductor device whose performance improves with minority carrier lifetime. Generally, the photoresponse of semiconductor solar cells is more favorable in larger grain size material. Material of a single crystal form free of grain boundaries is optimum. Semiconductor devices other than solar cells similarly benefit from use of single crystal materials.
To obtain single crystal semiconductor material for fabrication of devices, it is common practice in the semiconductor industry to melt or vaporize the material and then cool it in contact with a solid single crystal seed of the same or a similar material. As the molten or vaporized material cools, it solidifies epitaxially at the interface with the seed such that the single crystal morphology of the seed is propagated. In this way, the molten or vaporized material is converted to single crystal form.
In one version of the foregoing process, a seed is slowly withdrawn from a melt and supports a single crystal ingot. In another version, a laser is used to locally melt a layer of polycrystalline material deposited on an underlying single crystal seed substrate. In a third version, the material is chemically vapor deposited on a single crystal seed substrate usually at an elevated temperature. The main drawback of all these processes is the requirement for a single crystal seed.